ABSTRACT Phthalates are synthetic chemicals used in personal care products and in flexible plastic products. Along with their metabolites, phthalates are rapidly excreted in humans with half-lives of 5-24 hours. Exposure to phthalates is ubiquitous because of their widespread use and their ability to leach easily into the environment. Phthalates are endocrine-disrupting chemicals that may interfere with neuroendocrine pathways involved in neurodevelopment, including anti-androgenic activity and thyroid hormone homeostasis. The prenatal period is a particularly sensitive target for phthalate exposure due to rapid brain development. Previous studies of phthalates and neurodevelopment are limited by small sample size, cross-sectional design, and limited data on neurodevelopment. The primary objective of the proposed research is to investigate the association of prenatal exposure to phthalates with cognitive and behavioral development in childhood and adolescence. We will examine these relationships in the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS), a large, well-characterized longitudinal pre-birth cohort of mothers recruited 1999-2000 during pregnancy and their offspring. We will analyze existing data on maternal phthalate levels, measured twice during pregnancy (~13 and 26 weeks), in relation to the cohort's extensive data on neurodevelopment measured from age 7 years until adolescence for 336 children with exposure and outcome data. We will examine associations with the following outcomes: (1) general cognition, (2) executive function, (3) attention problems, externalizing behaviors and internalizing behaviors, and (4) social cognition. This study will take advantage of the CHAMACOS cohort's longitudinal data collected on teacher- and parent-rated scales as well as objective, performance-based tests. Additionally, this will be the first study to examine associations of phthalates with neurodevelopmental measures in adolescence, a critical time for manifestations of early life exposure effects. Results of this study, which will further our understanding of the long-term impact of early-life exposure to phthalates on neurodevelopment, have important policy implications given that widespread exposure to these chemicals in personal care products is modifiable and alternatives may be available during sensitive time windows, such as pregnancy.